DESCRIPTION (Applicant's abstract): Our overall objective is to synthesize two approaches to quantifying the biomechanical basis for the metabolic energy required for locomotion. The classical approach measures only the mechanical work performed and assumes that the metabolic energy required for isometric muscle activity is negligible. More recently, Kram and Taylor used a comparative zoological approach to test an alternative "cost of generating force" hypothesis. Their results indicate that the generation of muscular force to operate musculoskeletal springs is the dominant energy consuming process and that performing mechanical work has negligible cost. Empirical data supports aspects of each approach, but neither method can comprehensively explain all available data. The logical progression is to further develop the cost of generating force approach on human subjects and to deduce a quantitative, empirically based synthesis of the two approaches. We will test: Hypothesis 1. the metabolic cost of normal human walking is dominated by equal costs of performing external mechanical work on the center mass and of generating force to support body weight and Hypothesis 2. the metabolic cost of normal human running is determined primarily by the cost of generating vertical force to support body weight. Specific aims are to determine what fractions of the total metabolic energy demand are determined by the swinging of the legs, supporting body weight and performing mechanical work on the body center of mass. Experiments will systematically increase or decrease the forces and work required independently and measure the effect of these manipulations on the metabolic energy demand. Experimental manipulations involve simulated hypo-gravity, hyper-gravity, added mass, increased inertia and externally applied horizontal forces. These experiments will be the first that can independently vary and measure mechanical work and force generation. We will then be able to synthesize our results into a coherent overall picture for what muscular actions determine the metabolic cost. These experiments are only now feasible due to newly developed procedures (e.g., simulated hypo-gravity) and biomechanical equipment (e.g., a force measuring treadmill). Locomotion is energetically very expensive in individuals with metabolic (obesity), structural (lower extremity amputees), or gait disorders (cerebral palsy) and thus the energy cost itself can limit mobility. Understanding the basic mechanisms that determine the cost of locomotion will allow for more informed evaluation, treatment, and rehabilitation of these conditions.